Method of heat treating nitrided articles



Sept. 2, 1930. R. SERGESON METHb D OF HEAT TREATING NITRIDED ARTICLES Filed Sept. 20. 1929 o -a oa as 0-6 0-? Pad/0 7$ o O 0 O 2322 Jwwwtoz Pa d/ans Pater-t Gez-yeson tor of the nitridmg process,

Patented Sept. 2, 193 0 ROBERT SERGESON, OI HASSILLON, OHIO,-

REPUBLIC s'rnm. ooaroaarron, or'xounes'rown, 0310,

assiexoa, 3r MESH]: Ass mamas, ro'

minor) or near 'raaarmanrrnmnn narrows Application filed September 20, 1988. Serial'll'o. 884,108..

This invention relates in general to the nitriding of alloy steel articles and more particularl to a novel heat treatment for the nitride articles, whereby brittleness of the nitrided case is more'or less completely eliminated, and the toughness of the case increased to a considerable extent.

According to Fry, a-well known investigano nitridlng should be done at a temperature in excess of 1075 F., else the marginal layers of the nitrided zone willbe brittle and tendto peel off. For this reason, most nitriding has heretofore been. done below 1000 F., and

while, in most instances, the 'nitrided' zones have been found to be extremely hard, the zones are relatively thin and. the time required is often as much as 100 hours.

- Even when nitriding is done under the. aforesaid conditions, the nitrided case is often found tobe brittle- This brittleness manifests itself, for example, by the failure of the case when subjected to a too heavy impact when the supporting core'is upset;

To determine if the case is brittle, severaltests have been applied. One is to take a Vickers hardness reading on the nitrided surface after polishing with emery cloth.

Under this condition a brittle case will spall cone,

or flake ofl, while a non-brittle case will show a clean cut impression. A still more drastic test consists in taking a Rockwell no load reading on similar surfaces. A Rockwell? no load reading isobtained by using the Rockwell hardness tester with 'a diamond. but removing the usual 150 kilogram weight. .In this condition, the load onthe diamond is 60 kilograms. With this test, brittle cases will show much spalling,-while the non-brittle .case will not spall but may show some few radial cracks.

The toughness of the case may be determined by means of a Humphrey slow bend 4 machine, which records the amount of work done in breaking specimens which have been nitrided. The egree of bending, necessary to break the nitrided case is an indication of have discovered that the brittleness, of ordithe relative toughness of the case.

As a result of a series of experiments, I

nary nitrided cases, i. 0., cases formed by A. CORPORATION O! W nitriding at temperatures of from about 850 '1 F. to about 1050 F. for periods up to about 100 hours, may be almost completely eliminated or cured and the toughness thereof considerably increased by heating the articles to temperatures in excess of 1100 F. but below the lower critical oint of the core material,

and maintainin t e articles at such temperatures for perio s up to about one hour.

The lower critical point of ordinary chromium-molybdenum-aluminum nitriding steels is approximately 1420 F. While heatmg to temperatures in excess of thelower critical point of the steel does'not adversely affect the properties of the nitrided case to an appreciable degree, thephysical properties of the core material may be affected or lost, and hence, it is not advisable to heat to temperatures in excess of the lower critical point. The preferred temperature is about .1200 F. The nitrided articles are maintained at this temperature for eriods up to about one hour and referably or about one-half hour, but may e maintained at such temperature for longer periods, if desired.

In these expernnents, an alloy steel contammg 32% carbon, 56% manganese, .1 8%

silicon, 1.43% chromium, .18% molybdenum and 1.30% aluminum was employed.-

Specimens of this allo steel, after being nitrided at 975 F. for a out 48 hours wereheated at increasing temperatures starting from 1000 F. The temperatures 1000 F. and 1100 F. had little or no effect in removing the brittleness} At 1150 and 3 1200 F., how ever, the brittlenesswas completely gone.

Fig. 1. of the accompanying drawings shows Rockwell no load impression on a speci-' men of the aforementioned alloyfafter being nitrided, but, before heating. The marked spallin aof the nitridedcase maybe clearly I 7/ I Fig. 2 shows-a Rockwell no load impression on the same surface of the specimen 'as that shown in Fig. 1, but after the specimen had been heated to 1200 The clear cut impression shown in this figure indicates that brittleness has. been substantially elimdrawn'by the Humphrey machine and showinated. Both figures arereprciductions of microphoto'graphs taken at 100 magnifica- .tions.

Specimens of the same allog were alsoused for making the Humphrey slow bend tests to determine the toughness of the nitrided cases. Figs. 3 and 4 are stress-strain diagrams I gra hicall "the amount of work expended The method of m brea'ng t e specimens. The specimens forwhich the curve shownin Fig. 3 was obtained, were nitrided rbr 90 hours at 975' F. before being tested, while the specimens. ,for which the curve shown inF-ig; iwasxoh' taijned were nitrided for 90 .hours at 975 F.

and then heated to1200- F. and

temperature for one-half hour.

From these curves it is readilv-anparent held at this hours, and then heating the nitrided articles to a temperature in excess of 1100: Fibut below the lower critical point thereof.

8. The method of providing non-brittle, tough cases on steel articles, which consists in'nitriding the articles at a temperature of from about 850 F. to about 1050 F. for a period varying. from about 5 to about 100 hours, andthen heating the nitri'ded'articles;

to a temperature of about 1200 a In testimony whereof I afix my signa- ROBERT snRensoN;

thatthespecimens heated to 1200 F. after I nitriding show'a greater. bend and a greater area of work done," indicating agreater toughness of case. The area under the curves is a measure of the work done in breaking the specimens.

Claims:

. nessand increasing the toughness of the case point thereof.-

of nitrided articles which consists in heating theinitri'ded-articles to a temperatureinexan oess or 1100 F.- butfbelow the lower critical point-thereof, I l s 2. ?Themethodof decreasing the brittleness and increasing the toughness of the case of nitridhdarticles which consists in heating the nitrided articles to a temperature of about 3. "The method of prouidin non-brittle,

thenitride articles to a temperature of about 5.- The method of providing-non-brittle,

a heating 'the nitrided articles toa tough cases on steel articles, which consists innitriding the articles at a tem erature'of from about 850 Fltoabout '1050' and then temperature in excess of 1'100 F.*,bi1 t below .the lower'critical point of thejsteelr' "6. The method ofproviding non-brittle,

, tough cases on steel articlesywhich consists in nitriding-the"arti cles at a tem erature of from about .850 F. to about 1050 r I, andithen heating the nitrided art cles toa temperature 50- I v v ,7; The jme'thod of of 'ahout 1200. F. V

toughcases on steel articles,

which consists in nitriding the articles at a from about. 8502mm about 1050 F. for a decreasing the brittleproviding lion-brittle,

temperature of 'per iodvarying fiemjabqut 'e to about 1 00 i 

